X-ray backscatter (BX) imaging has been used since the 1980's for detecting concealed contraband, such as drugs, explosives, and weapons. The BX imaging technique differs in fundamental ways from the traditional transmission (TX) x-ray method of searching for contraband. The TX method works by creating an image of the x-rays that are transmitted through the target. Transmission images are created by a fan or cone beam of x-rays that pass through the target to a pixelated x-ray detector. The size of the individual detector elements determines the pixel resolution in the transmission image.
In contrast to TX images, backscatter images are created by scanning the target with a pencil beam of x-rays and measuring the intensity of the backscattered x-rays as a function of the position of the pencil beam on the target. Both x-ray scanners and backscatter detectors have been used on mobile platforms.
Scanning pencil beams of x-rays are created in several distinct ways, which have in common the creation of a fan beam of x-rays that is periodically cut (chopped) by a rotating slot or aperture created by a variety of means. The spatial resolution in the image is determined by the cross sectional area of the resulting scanning beam at the point that it strikes the target object being imaged.